Thermometric ring oscillators are temperature sensors. Other temperature sensors include thermometers, thermocouples, thermistors, resistance temperature detectors, oscillate crystals and junction diodes, among others. Typically these sensors are placed in close proximity to the point where a temperature measurement is desired, and are connected by wires to electronic components that process the output of the sensors. In many applications, however, the wires connecting to the sensors are inconvenient. In a manufacturing production line, for example, the sensors may be moving while the measured objects remain stationary. In other applications, a temperature sensing point may be difficult to access, such as inside a nuclear reactor or other harsh or sensitive environment, where wired connections may be impractical.
Thermometric ring oscillators typically comprise an odd number of inverters connected in series to each other. The output of one inverter is connected to the input of an adjacent inverter, and the output of the last inverter is connected to the input of the first inverter. U.S. Pat. No. 5,418,499, among other references, discloses that such ring oscillators oscillate at a range of known frequencies and that ring oscillators have been used as internal clocks in computers and other electronic equipments. The '499 patent also discloses a variable ring oscillator that in response to a switching signal less than all of the inverters in the ring can be connected together to control the output frequency of the oscillation signal. Hence, a wider range of frequency band can be utilized. The oscillate frequencies of ring oscillators, however, suffer from performance variations due to the actual voltages applied to the inverters and to the operating temperatures of the electronic equipments.
Constructing and predicting the operating frequencies of ring oscillators are known in the art. See, e.g., “A Method to Derive an Equation for the Oscillation Frequency of a Ring Oscillator,” by Stephen Docking and Manoj Sachdev, published in the IEEE Transactions on Circuits and Systems-I: Fundamental Theory and Applications, Vol. 50, No. 2, February 2003, pp. 259–264. It is also known in the art that two ring oscillators, seemingly identical in construction, may exhibit differences in respective oscillator frequencies due to differences in supply voltages and minute differences in fabrication, even if the oscillators are monolithically realized and cut from the same wafer. See, e.g., “A CMOS Circuit for Real-Time Chip Temperature Measurement,” by Steven R. Boyle and Raymond A. Heald, published in Compcon Spring '94, Digest of Papers, Feb. 28-Mar. 4, 1994, pp. 286–291.
As disclosed in U.S. Pat. Nos. 4,448,549, 4,549,818 and 4,658,407, the temperature-induced frequency variations in ring oscillators can be used to measure temperature. As illustrated in FIG. 1, U.S. statutory invention registration No. H1744 discloses a temperature measuring device comprising ring oscillator 10 made from an odd number of inverters 12 having a nominal oscillate frequency, and oscillator 10 is positioned at a location where temperature is to be measured. The ring oscillator emits electromagnetic radiation to an antenna located at a convenient distance from the ring oscillator. The antenna transforms the electromagnetic radiation into an electrical signal. A receiver receives the electrical signal and measures the frequency of the electrical signal to determine the corresponding temperature. The temperature may then be visually monitored from a display or electronically monitored by other devices. This ring oscillator functions both as a temperature sensor and as a wireless transmitter of the frequency representing the temperature to a remote receiver.
However, the prior art does not disclose a wireless ring oscillator temperature sensor that can transmit its frequency over multiple channels.